We wish to continue our investigation of the control of collagenolytic activity obtained from mammary carcinoma and osteosarcoma cells. We have isolated a low molecular weight protein from cartilage which inhibits collagenase. We wish to pursue the idea that this may represent a mechanism of in vivo control of the invasiveness of tumors via modulation of collagenolytic activity. The information we hope to obtain may provide some insight into the regulation of collagen degradation in normal and disease states.